As required data rates increase in computers and data switches and the like optical signal carriers are being used in place of or as well as conventional copper signal carriers. Optical printed circuit boards (PCBs) have optical waveguides that are used for the transmission of light signals between components, as well as or instead of conventional copper conductors. Typically, an optical PCB consists of a base or support layer. In areas of the optical PCB where optical waveguides are required, a lower optical cladding layer is provided usually of uniform thickness. On top of this, a layer of optical core material is laid down. The optical core material has a higher refractive index than the cladding layer and will eventually form one or more optical waveguides on the optical PCB.
In a known process used for making optical PCBs, the core layer is laid down in liquid form, e.g. as a curable liquid polymer. A mask having openings corresponding to the desired shape of the waveguides is arranged over the liquid polymer and the entire resultant structure is then irradiated with electromagnetic radiation of suitable wavelength. Thus, in regions of the mask which are open, the liquid polymer is cured. In other regions, the polymer remains liquid. The mask is removed and the remaining liquid polymer can be washed away leaving the desired pattern of optical waveguides.
The remaining core material is typically arranged in patterns of channels which are arranged in some manner so as to be able to couple optical signals between components on the optical PCB when the components are arranged thereon. Last, an upper cladding layer is laid down, so that the channels of core material are completely surrounded by cladding material, and therefore are able to function as optical waveguides. Waveguides can guide light even if surrounded by air, because air has a lower refractive index than the core material. It is preferable that core waveguides are surrounded by a material of uniform refractive index, so there is no asymmetry in boundary conditions on some facets.
Tapered optical waveguides are known. See, for example, the disclosure of U.S. Pat. No. 7,039,289, discussed in detail below. Typically in a tapered waveguide at the ingress end, i.e. the end at which light is launched into the waveguide, the waveguide diameter or rather the area of the input interface depending on waveguide cross-section is made as large as possible to allow for misalignment of the light source with the input interface. In contrast, at the egress end, at which light which has propagated through the waveguide is launched into an optical receptor, e.g. a lens, photodiode, optical fibre etc, waveguide diameter or rather the area of the output interface depending on waveguide cross-section is made as small as possible to maximise the amount of light caught by the receptor.
As light propagates along a tapered waveguide significant signal loss can occur due to modal expulsion which occurs since the number of optical modes that can be supported by a waveguide is dependent in part on the boundary conditions of the waveguide. The larger the cross-section area, the higher the number of modes that can be supported. Therefore as the cross sectional area decreases along the taper, the number of modes that can be supported decreases correspondingly.
To address this it has been suggested that the refractive index of the optical core of the waveguide be changed to compensate for the modal expulsion due to the varying diameter. It is known that the number of supported modes also depends on the refractive index difference between the waveguide core and its cladding. Therefore, in theory, modifying the refractive index of the core along the length of a tapered waveguide should be able to ensure that modal expulsion is minimised or even eliminated.
WO-A-95/23037 discloses a method and apparatus by which micro-optical components can be printed directly onto an optical substrate or an active device to create optical circuit elements as well as micro-optical components such as lenses and waveguides. The method provides a means for depositing a wide range of materials in a wide variety of shapes for fabricating a range of passive and active micro-optic devices. As shown in and described with reference to FIG. 9, a waveguide can be formed by dropping a plurality of drops of optical material in a desired pattern.
U.S. Pat. No. 7,039,289 discloses a process for fabricating integrated optics devices. A photosensitive sol-gel glass material including a volatile photosensitiser is prepared and a film of the material is laid down prior to patterning to form a desired device by exposure to curing radiation. A variety of passive and active integrated optic devices may be manufactured using the disclosed method. One example is a tapered waveguide. With reference to FIG. 31 of this document it is disclosed that to maintain the number of propagation modes within the tapered waveguide, the refractive index of the expander is decreased as its width increases.
A problem with the disclosed method is that it is extremely complex and relies on the use of glass which can be difficult to work with. A simple and reliable method of producing a waveguide, such as a tapered waveguide, is required. When used herein the term “tapered waveguide” refers to a waveguide having a tapered optical core or the tapered core itself.